Ion-selective potentiometry has proved to be highly successful as an important method for determining free ion concentrations since various, more or less selective electrodes for certain cations and anions had become commercially available in the 1960s and since the publication of the first monographs K. Cammann, "Das Arbeiten mit ionenselektiven Elektroden" 1. Auflage, Springer Verlag, Berlin, 1973 on this analytical method, which is called the renaissance of the pH-measuring technique. From the viewpoint of the measuring technique, ion-selective potentiometry extensively corresponds to the conventional pH-measuring technique based on the extremely selective pH-glass membrane electrodes. Only an ordinary constant-potential reference electrode (e.g., one based on calomel or Ag/AgCl), as well as a correspondingly high-ohmic millivoltmeter (pH meter) with an accuracy of .+-.0.1 mV are needed for the accurate determination of certain ion concentrations, besides the corresponding measuring electrode that is selective for the ion to be determined. Ion concentrations between the saturation limit and a few ppb can be reliably determined with a measuring arrangement of such a simple design. The quality of the analytical results (i.e., the accuracy of the analysis) is usually determined by the ion-selective measuring electrode. It operates selectively and reversibly as a true chemosensor, i.e., it is able to indicate changes in concentrations in both directions, unlike test bars or test tubes. This is an absolutely necessary requirement for continuous on-line measurements.
The relationship between the measuring chain voltage and the concentration of an ion to be determined is described, to a sufficient accuracy, by an expanded Nernst equation: EQU U=U.degree.+R T/z.sub.M F ln (c.sub.M +K.sub.M-I c.sub.I.sup.zM/zI)
in which U=measuring chain voltage,
U.sub.o =measuring chain voltage at ln=1,
R=Faraday's constant,
T=temperature,
zM=value and charge of the measured ion,
z.sub.I =value and charge of the interfering ion,
c.sub.M =concentration of the measured ion,
c.sub.1 =concentration of the interfering ion, and
K.sub.M-I =selectivity coefficient between the measured ion and the interfering ion.
It is assumed in this analysis function that the ionic strength does not change between calibration and measurement. A slight preparation of the sample solutions by dilution with an ionic strength-adjusting solution, which may also contain masking chemicals for particularly interfering ions, may be necessary under certain circumstances. Flow injection analysis (FIA) is an especially elegant type of sample preparation, which can be automated. Only a few microliters of sample are injected here into a carrier flow of a suitable electrolyte, which flows past the measuring chain (flow-measuring cell) at a rate in the range of mL/minute, and the peak height that develops is used for quantification. Depending on the effective flow rate of the basic (carrier) electrolyte and the dead volume of the potentiometric measuring chain, it is possible to inject 60 samples per minute, which is tantamount to a quasi-continuous measurement (time constant ca. 1 minute). The advantage of the combination of the prior-art flow injection technique with ion-selective potentiometry was soon recognized J. Ruzicka, E. H. Hansen: "Flow Injection Analysis" Wiley, New York, 1981. The carrier liquid flow (carrier solution) can thus be optimized for the measuring electrode in question. By selecting a suitable composition of this basic electrolyte, it is also possible to build a measuring chain without conversion with this combination, which also offers advantages in terms of design engineering, besides increased accuracy (no fluctuating diffusion potential at the reference electrode-electrolyte/measuring solution phase boundary), because it becomes unnecessary to provide an additional reference electrode electrolyte solution for prolonged on-line operation, or such solution is provided by the FIA carrier solution.
The usage time that can be achieved under real conditions is very important when ion-selective potentiometry is used for continuous measurement analogously to the pH flow fixtures or within the framework of an FIA arrangement. Unlike other membrane electrodes with renewing or renewable surfaces, the class of the so-called liquid membrane electrodes has only a limited service life under these measurement conditions in its most significant design with a polymer-solidified, liquid-like membrane (mostly on the basis of PVC, siloxane, polyacrylamide, etc.). The service life, i.e., the guarantee of the central electroanalytical criteria, such as selectivity, detection limit, Nernst behavior, stability, and speed of indication, of such polymer membrane electrodes is generally restricted by two effects:
a) The bleeding of selectivity-generating membrane components (plasticizing agent and electroactive compound) and
b) Impairment of the ratio of the standard exchange current density of measured ions to interfering ions because of the slow increase in the concentration of interfering ions in the membrane phase.